1. Field of the Invention
The present invention concerns a method to image a particle that is located in an examination subject as well as a magnetic resonance device for this. In particular, the invention concerns the imaging of a particle that causes a magnetic interference field in an applied basic magnetic field during the acquisition of magnetic resonance data.
2. Description of the Prior Art
Magnetic resonance tomography is a wide-spread method for graphical representation of structures inside a body of a patient. To generate a magnetic resonance signal, in general protons of hydrogen molecules are excited that have been organized in a prepared magnetic state. Decay of this excitation induces a magnetic resonance signal in an acquisition coil. The magnetic resonance signal is thus dependent on (among other things) the density of the protons of the hydrogen molecules. A low magnetic resonance signal thus is obtained from regions to be imaged that have a low proton density (for example from air-filled regions or from bones), which leads to representation of the corresponding regions as a dark location in the magnetic resonance images. However, such dark locations in magnetic resonance images can also be caused by other mechanisms, for example by local magnetic fields that are caused by magnetically active substances. These local magnetic fields lead to a dephasing of the excited magnetization and thus generate what is known as a hypointense contrast.
This hypointense contrast can be utilized to show probes in the form of particles into which the magnetically active substances are integrated. Such particles have a number of uses in clinical routines and in research, for example in the field of pharmaceutical carrier systems. Due to their magnetic activity, these magnetically active substances can cause a magnetic interference field, for example a dipole field, upon the application of the basic magnetic field of a magnetic resonance measurement, so they are imaged (mapped) with hypointense contrast and consequently can be localized. Particularly in T2*-weighted gradient echo sequences, the disruption of the homogeneous basic magnetic field leads to a signal loss. In spin echo sequences, these interferences additionally lead to susceptibility artifacts. A problem in this type of imaging of particles is that the hypointense image regions cannot be unambiguously associated with the particles since—as described above—there are multiple causes for a poor signal and the corresponding dark image regions.
Therefore, methods have been proposed that generate a hyperintense contrast. These methods utilize the magnetic dipole field of the particles that produces both magnetic field gradients and a change of the proton Larmor frequency in the immediate environment. A resonance region of proton signals of protons bound in hydrogen molecules is expanded by the interference field.
A spin echo-based, spectral method for hyperintense measurement of the magnetic interference particles is described in “Positive contrast visualization of iron oxide-labelled stem cells using inversion-recovery with ON-resonant water suppression (IRON)”, M. Stuber et. al., Magn. Reson. Med, 58:1072-1077, November 2007. Proton signals of protons bound in fat and/or water molecules are hereby inverted and/or suppressed via spectrally selective inversion pulses before a beginning of a magnetic resonance measurement. A T1 dependency is created that leads to the situation that substances with an inhomogeneous time constant T1 cannot be sufficiently suppressed. However, these methods have the disadvantage that the inversion pulses in the spectral space have only an approximation of an ideal box shape and are fuzzy specifically at edges. This leads to the situation that for the most part either a spectrally broadband saturation takes place, such that non-resonant signals are saturated as well, or a narrowband saturation takes place such that too much signal of the undisrupted water protons remains.
An additional spin echo-based method is described in “Positive contrast magnetic resonance imaging of cells labeled with magnetic nanoparticles”, C. H. Cunningham, Magn. Reson. Med., 53:999-1005, May 2005, in which only selected non-resonant regions to the left and/or right around the proton signal of protons that are bound in water molecules are excited by an excitation pulse. However, this excitation pulse is very fuzzy in the spectral space.